U.S. patent application number 15/608082 was filed with the patent office on 2018-02-08 for method for manufacturing electrode for hydrogen production using tungsten carbide nanoflake and electrode for hydrogen production manufactured thereby.
The applicant listed for this patent is KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY. Invention is credited to Young Joon BAIK, Doo Seok JEONG, Inho KIM, Young-Jin KO, Kyeong Seok LEE, Wook Seong LEE, Jong-Keuk PARK.
Application Number | 20180038003 15/608082 |
Document ID | / |
Family ID | 61071558 |
Filed Date | 2018-02-08 |
United States Patent
Application |
20180038003 |
Kind Code |
A1 |
LEE; Wook Seong ; et
al. |
February 8, 2018 |
METHOD FOR MANUFACTURING ELECTRODE FOR HYDROGEN PRODUCTION USING
TUNGSTEN CARBIDE NANOFLAKE AND ELECTRODE FOR HYDROGEN PRODUCTION
MANUFACTURED THEREBY
Abstract
A method for manufacturing an electrode for hydrogen production
using a tungsten carbide nanoflake may include: forming a tungsten
carbide nanoflake on a nanocrystalline diamond film by means of a
chemical vapor deposition process in which hydrogen plasma is
applied; and increasing activity of the tungsten carbide nanoflake
to a hydrogen evolution reaction by removing an oxide layer or a
graphene layer from a surface of the tungsten carbide nanoflake.
Since an oxide layer and/or a graphene layer of a surface of
tungsten carbide is removed by means of cyclic cleaning after
tungsten carbide is formed, hydrogen evolution reaction (HER)
activity of the tungsten carbide may be increased, thereby
enhancing utilization as a catalyst electrode.
Inventors: |
LEE; Wook Seong; (Seoul,
KR) ; KO; Young-Jin; (Seoul, KR) ; BAIK; Young
Joon; (Seoul, KR) ; PARK; Jong-Keuk; (Seoul,
KR) ; LEE; Kyeong Seok; (Seoul, KR) ; KIM;
Inho; (Seoul, KR) ; JEONG; Doo Seok; (Seoul,
KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
KOREA INSTITUTE OF SCIENCE AND TECHNOLOGY |
Seoul |
|
KR |
|
|
Family ID: |
61071558 |
Appl. No.: |
15/608082 |
Filed: |
May 30, 2017 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C23C 16/32 20130101;
C23C 16/503 20130101; C25B 11/02 20130101; Y02E 60/366 20130101;
B82Y 40/00 20130101; Y02E 60/36 20130101; C25B 11/0447 20130101;
B82Y 30/00 20130101; C23C 16/56 20130101; C25B 11/0415
20130101 |
International
Class: |
C25B 11/04 20060101
C25B011/04; C23C 16/32 20060101 C23C016/32; C25B 11/02 20060101
C25B011/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 8, 2016 |
KR |
10-2016-0100552 |
Claims
1. A method for manufacturing an electrode for hydrogen production
using a tungsten carbide nanoflake, comprising: forming a tungsten
carbide nanoflake on a nanocrystalline diamond film by means of a
chemical vapor deposition process in which hydrogen plasma is
applied; and increasing activity of the tungsten carbide nanoflake
to a hydrogen evolution reaction by removing an oxide layer or a
graphene layer from a surface of the tungsten carbide
nanoflake.
2. The method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to claim 1,
wherein in the chemical vapor deposition process in which hydrogen
plasma is applied, in a state where a substrate having a
nanocrystalline diamond film is provided on an anode in a chamber
and a surface-carburized tungsten cathode is provided at a location
upwardly spaced apart from the substrate, hydrogen plasma is
applied into the chamber.
3. The method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to claim 1,
wherein the step of forming a tungsten carbide nanoflake includes
controlling the degree of supersaturation at a growth front of
tungsten carbide so that the tungsten carbide grows to have a
nanowall structure.
4. The method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to claim 3,
wherein the step of controlling the degree of supersaturation at a
growth front of tungsten carbide includes controlling a flux of
growth species by adjusting a process temperature of the chemical
vapor deposition process in which hydrogen plasma is applied.
5. The method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to claim 3,
wherein the step of controlling the degree of supersaturation at a
growth front of tungsten carbide includes controlling a flux of
growth species by adjusting a discharge voltage and current which
is applied for generating hydrogen plasma in the chemical vapor
deposition process in which hydrogen plasma is applied.
6. The method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to claim 2,
wherein the surface-carburized tungsten cathode has a carbonization
layer formed to a predetermined depth from the surface of the
tungsten carbide nanoflake by exposing the tungsten cathode to a
carbon environment of a predetermined temperature, and the
carbonization layer forms a WC.sub.x, structure.
7. The method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to claim 1,
after the increasing activity of the tungsten carbide nanoflake to
a hydrogen evolution reaction, further comprising: forming a
protection film to partially cover the surface of the tungsten
carbide nanoflake.
8. An electrode for hydrogen production using a tungsten carbide
nanoflake, comprising: an electrode body comprising a
nanocrystalline diamond film located on a substrate, and a tungsten
carbide nanoflake located on the nanocrystalline diamond film; and
a protection film configured to partially cover the electrode
body.
9. The electrode for hydrogen production using a tungsten carbide
nanoflake according to claim 8, wherein the tungsten carbide
nanoflake has a two-dimensional nanostructure arranged on the nano
crystalline diamond film.
10. The electrode for hydrogen production using a tungsten carbide
nanoflake according to claim 9, wherein the tungsten carbide
nanoflake has a nanowall structure.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to Korean Patent
Application No. 10-2016-0100552, filed on Aug. 8, 2016, and all the
benefits accruing therefrom under 35 U.S.C. .sctn. 119, the
contents of which in its entirety are herein incorporated by
reference.
BACKGROUND
1. Field
[0002] Embodiments relate to a method for manufacturing an
electrode for hydrogen production using a tungsten carbide
nanoflake, and an electrode for hydrogen production manufactured by
the method.
2. Description of the Related Art
[0003] Hydrogen molecule (H.sub.2) is gathering attention as an
energy carrier. As a fuel, hydrogen has a greatest energy density
per unit mass and generates only water as a side product. To the
contrary, a carbon-based fuel generates carbon dioxide as a side
product. In addition, hydrogen is an infinite element on the earth,
and thus it is not needed to check its residual amount, different
from carbon-based fuels. However, hydrogen is not present alone in
nature but is found in a compound, and thus, the technique to
produce hydrogen from compounds is of great importance.
[0004] In these days, most hydrogen is produced by means of a steam
reforming process using fossil fuels. However, carbon dioxide
produced by reacting steam with hydrocarbon still causes a problem,
and thus researches have been focused on hydrogen production
techniques not generating carbon dioxide. Among them, electrolysis
of water is a hydrogen production technique which is
environmentally clean and reproducible. Electrochemical water
separation needs two half-cell reactions, namely hydrogen evolution
reaction (HER) and oxygen evolution reaction (OER).
[0005] In the HER, platinum serves as a substance with a greatest
catalytic activity. However, platinum is one of small-deposit
materials, and thus it is difficult to meet the global energy
consumptions using platinum. Therefore, it is needed to develop a
catalyst electrode using an element which may replace platinum and
is also rich on the earth. In particular, the necessary conditions
of a heterogeneous catalyst for substituting platinum may include
(1) low electrochemical overpotential and (2) stability in
operation.
[0006] Among many candidate materials, tungsten carbide which is a
kind of metal carbides has received great attention, and tungsten
carbide is known as being very excellent in thermal and
electrochemical stability. In particular, tungsten carbide has a
D-band density of state (DOS) similar to platinum and thus is
regarded as a candidate for substituting platinum in various
electrocatalytic reactions of HER or the like.
[0007] However, conventional techniques for composing tungsten
carbide mostly causes sintering, and in many cases, a passivating
oxide film serving as an insulator in a solution state is formed on
a surface on tungsten carbide. In particular, the passivating oxide
film is easily formed on the surface of tungsten carbide in an
acidic medium. Due to this drawback, it is being studied to modify
a surface of tungsten carbide by means of preprocess or
nanoparticles on which the passivating oxide film is not easily
formed. However, most researches are focused on nanoparticles, and
thus there is a problem in long-term stability.
SUMMARY
[0008] An aspect of the present disclosure is directed to providing
a method for manufacturing an electrode for hydrogen production,
which may have an improved hydrogen evolution reaction (HER) by
removing an oxide layer and/or a graphene layer on a surface of
tungsten carbide by means of cyclic cleaning after forming a
tungsten carbide nanoflake as an electrode body, and an electrode
for hydrogen production manufactured by the method.
[0009] Another aspect of the present disclosure is directed to
composing various kinds of nanostructures, particularly tungsten
carbide of a nanowall or nanocrystalline structure, by adjusting
the degree of passivating oxide film at the growth of tungsten
carbide to control an arrangement of the nano structure of tungsten
carbide.
[0010] A method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to an
embodiment comprises: forming a tungsten carbide nanoflake on a
nanocrystalline diamond film by means of a chemical vapor
deposition process in which hydrogen plasma is applied; and
increasing activity of the tungsten carbide nanoflake to a hydrogen
evolution reaction by removing an oxide layer or a graphene layer
from a surface of the tungsten carbide nanoflake.
[0011] In an embodiment, in the chemical vapor deposition process
in which hydrogen plasma is applied, in a state where a substrate
having a nanocrystalline diamond film is provided on an anode in a
chamber and a surface-carburized tungsten cathode is provided at a
location upwardly spaced apart from the substrate, hydrogen plasma
is applied into the chamber.
[0012] In an embodiment, the step of forming a tungsten carbide
nanoflake may include controlling the degree of supersaturation at
a growth front of tungsten carbide so that the tungsten carbide
grows to have a nanowall structure.
[0013] In an embodiment, the step of controlling the degree of
supersaturation at a growth front of tungsten carbide may include
controlling a flux of growth species by adjusting a process
temperature of the chemical vapor deposition process in which
hydrogen plasma is applied.
[0014] In an embodiment, the step of controlling the degree of
supersaturation at a growth front of tungsten carbide may include
controlling a flux of growth species by adjusting a discharge
voltage and current which is applied for generating hydrogen plasma
in the chemical vapor deposition process in which hydrogen plasma
is applied.
[0015] In an embodiment, the surface-carburized tungsten cathode
may have a carbonization layer formed to a predetermined depth from
the surface of the tungsten carbide nanoflake by exposing the
tungsten cathode to a carbon environment of a predetermined
temperature, and the carbonization layer may form a WC.sub.x,
structure.
[0016] In an embodiment, the method for manufacturing an electrode
for hydrogen production using a tungsten carbide nanoflake may
further include forming a protection film to partially cover the
surface of the tungsten carbide nanoflake.
[0017] An electrode for hydrogen production using a tungsten
carbide nanoflake according to an embodiment comprises: an
electrode body comprising a nanocrystalline diamond film located on
a substrate, and a tungsten carbide nanoflake located on the
nanocrystalline diamond film; and a protection film configured to
partially cover the electrode body.
[0018] In an embodiment, the tungsten carbide nanoflake may have a
two-dimensional nanostructure (for example, a nanowall or
nanocrystalline structure) arranged on the nanocrystalline diamond
film.
BRIEF DESCRIPTION OF THE DRAWINGS
[0019] FIG. 1 is a flowchart for illustrating a method for
manufacturing an electrode for hydrogen production using a tungsten
carbide nanoflake according to an embodiment.
[0020] FIG. 2 is a schematic view showing an electrode for hydrogen
production using a tungsten carbide nanoflake according to an
embodiment.
[0021] FIG. 3 is a graph comparatively showing linear sweep
voltammetry (LSV) curves of the tungsten carbide nanoflake composed
according to embodiments and a platinum (Pt) electrode.
[0022] FIGS. 4A to 4D show an X-ray photoelectron spectroscopy
(XPS) spectrum of a tungsten carbide nanoflake composed according
to embodiments.
[0023] FIGS. 5A to 5D show an XPS spectrum of a tungsten carbide
nanoflake composed according to other embodiments.
[0024] FIG. 6 shows a scanning electron microscope (SEM) image of a
tungsten carbide nanowall film composed according to an
embodiment.
[0025] FIGS. 7A to 7D show an XPS spectrum of a tungsten carbide
nanoflake composed according to other embodiments.
[0026] FIGS. 8A and 8B is a graph showing polarization curves
before and after a durability test of the tungsten carbide
nanoflake according to the embodiments of FIG. 7.
[0027] FIG. 9 is a high-resolution transmission electron microscope
(HR-TEM) image of a tungsten carbide nanowall composed according to
an embodiment, after cleaning.
[0028] FIGS. 10A and 10B show a Tafel plot and a turnover frequency
(TOF) of a tungsten carbide nanoflake composed according to
embodiments.
DETAILED DESCRIPTION
[0029] Hereinafter, configurations and features of the present
disclosure will be described based on embodiments, but these
embodiments just illustrate the present disclosure and do not limit
the present disclosure.
[0030] FIG. 1 is a flowchart for illustrating a method for
manufacturing an electrode for hydrogen production using a tungsten
carbide nanoflake according to an embodiment.
[0031] Referring to FIG. 1, a nanoflake of tungsten carbide (WC)
may be made on a nanocrystalline diamond (NCD) film by means of a
plasma-assisted chemical vapor deposition (PACVD) process in which
hydrogen plasma is applied (S1). In an embodiment, a DC-PACVD using
a direct current is applied.
[0032] More specifically, an anode on which a substrate is placed
is provided in a chemical vapor deposition chamber, and a tungsten
cathode is provided at a location upwardly spaced apart from the
substrate. Since NCD is very stable in the presence of hydrogen
atoms or under conditions such as acidic or alkali environments,
the NCD is used as a substrate for the growth of tungsten carbide.
If a power is applied to the anode and the cathode together with
supplying hydrogen into the chamber, hydrogen plasma is generated.
At this time, the tungsten cathode and the NCD serve as sources of
growth species, namely a source of tungsten atom and a source of
carbon atom, respectively. As a result, a nanoflake of tungsten
carbide is formed on the NCD film.
[0033] In other words, in order to form tungsten carbide of a
two-dimensional nanostructure according to embodiments of the
present disclosure, 1) a chemical vapor deposition process is
required, and also 2) it is required to apply hydrogen plasma
during the chemical vapor deposition process.
[0034] As described above, in the chemical vapor deposition
process, growth species (tungsten atom, carbon atom) is supplied
from the tungsten cathode and the NCD to form tungsten carbide of a
nanowall structure on the NCD.
[0035] In an embodiment, a tungsten carbide nanoflake having a
two-dimensional nanostructure is formed. The two-dimensional
nanostructure may be a structure where nanoflakes are arranged in a
vertical direction with regard to the plane of the substrate, for
example a nanowall structure or a nanocrystalline structure.
[0036] When forming the tungsten carbide, the NCD film somewhat
serves as a source of carbon atom. However, for the growth into a
nanowall structure or a nanocrystalline structure with a vertical
arrangement, carbon should be supplied on the NCD film. For this
purpose, the tungsten cathode needs to employ a surface-carburized
tungsten cathode. The surface-carburized tungsten cathode is
obtained by carbonizing a tungsten cathode and has a carbonization
layer to a predetermined depth from the surface. The carbonization
layer is made of WC.sub.x, material, and the carbonization layer
may be formed by exposing the tungsten cathode to a methane gas
environment of a predetermined temperature. The surface-carburized
tungsten cathode, or particularly the carbonization layer, serves
as a source of tungsten atom and carbon atom when tungsten carbide
of a nanowall or nanocrystalline structure is formed.
[0037] Meanwhile, in the present disclosure, it is premised that
hydrogen plasma is applied during the chemical vapor deposition
process. If source plasma is generated at high pressure of 100 Torr
or above, hydrogen plasma makes a physicochemical interaction with
the surface-carburized tungsten cathode and gives a driving force
so that the tungsten atom and the carbon atom move onto the NCD
film at the carbonization layer of the tungsten cathode.
[0038] Since the hydrogen plasma physically contacts the tungsten
cathode, the above process is somewhat similar to sputtering which
is a physical deposition method. However, in the sputtering, a
pressure in the chamber should be very low, and argon (Ar) heavier
than hydrogen is used in the sputtering process. Meanwhile, the
hydrogen plasma of the present disclosure has very low sputtering
efficiency due to light weight, and the pressure in the chamber to
which the hydrogen plasma is applied is also set higher than in the
sputtering process. Thus, in the present disclosure, the process in
the hydrogen plasma contacts the tungsten cathode should not be
limited to sputtering.
[0039] In the above description, the term "physicochemical
interaction of the hydrogen plasma and the tungsten cathode"
reflects the above situation. In the present disclosure, the
hydrogen plasma plays a role of relaxing and dissolving a chemical
bond of the tungsten cathode, particularly the carbonization layer
(WC.sub.x), and accordingly, it is interpreted as the tungsten atom
and the carbon atom of the carbonization layer are diffused in a
vapor form onto the NCD film. For reference, the relaxing and
dissolving of the chemical bond of the carbonization layer
(WC.sub.x) is naturally influenced by the temperature (600 to
800.degree. C.) in the chamber.
[0040] In addition, the plasma applied in the present disclosure is
made of pure hydrogen, and this also makes it possible to supply
the tungsten atom and the carbon atom of the carbonization layer.
According to another study of the inventor of the present
disclosure (H. J. Lee, H. Jeon and W. S. Lee, J. Appl. Phys., 2011,
109, 023303), it has been revealed that a binary alloy is not
composed under a mixed gas environment of methane and hydrogen due
to the inert property of tungsten. In this consideration, in the
present disclosure, it is judged that not only the tungsten atom
but also the carbon atom is deviated and moved in the carbonization
layer since pure hydrogen plasma without carbon (methane) is
used.
[0041] Meanwhile, a geometric shape of the tungsten carbide formed
on the NCD film is determined depending on the degree of
supersaturation on a growth front when the tungsten carbide grows.
If the degree of supersaturation is relatively low, the tungsten
carbide forms a nanowall structure, and if the degree of
supersaturation is relatively high, the tungsten carbide forms a
film shape on the NCD.
[0042] The degree of supersaturation is determined depending on a
flux of growth species (or, a growth species flux) supplied onto
the NCD film at the carbonization layer of the tungsten cathode.
Therefore, if the growth species flux is relatively high, tungsten
carbide in a film form is formed due to high supersaturation, and
if the growth species flux is relatively low, tungsten carbide of a
nanowall structure is formed due to low supersaturation.
[0043] Factors determining the growth species flux include 1) a
generation rate of the growth species at the carbonization layer
and 2) a diffusion rate of the generated growth species. The factor
1) may be controlled by adjusting a discharge voltage and current
applied to the anode and the tungsten cathode, and the factor 2)
may be controlled by adjusting a process temperature in the
chamber. If the discharge voltage and current is high or the
process temperature is high, the growth species flux increases, and
resultantly, the growth front forms a high supersaturation state so
that tungsten carbide of a film form grows. Meanwhile, if the
discharge voltage and current is low or the process temperature is
low, the growth species flux decreases, and resultantly, the growth
front forms a low supersaturation state so that tungsten carbide of
a nanowall form grows.
[0044] In a relatively low supersaturation state, the nanowall
structure is formed, and in a relatively high supersaturation
state, the film structure is formed, due to the following
reasons.
[0045] In a relatively low supersaturation state, since the growth
species flux is low, secondary nucleation on the growth front is
minimized during a growth process to vertically grow in a
single-crystal form, thereby forming a nanowall structure.
Meanwhile, in a relatively high supersaturation state, since the
growth species flux is high, secondary nucleation is repeatedly
performed on the growth front to disturb vertical growth, thereby
inevitably forming a poly-crystal film finally.
[0046] In an embodiment, based on the above principle, by adjusting
a discharge voltage and a discharge current applied to the anode
and the tungsten cathode and/or a process temperature in the
chamber, the growth species flux is controlled so that the tungsten
carbide nanoflake has a two-dimensional nanostructure such as a
nanowall structure.
[0047] Next, by means of a cleaning process through a plurality of
cycles, the oxide layer or the graphene layer is removed from the
surface of the tungsten carbide nanoflake (S2). The oxide layer or
the graphene layer is removed by cleaning in order to enhance
catalytic activity of the tungsten carbide nanoflake at a hydrogen
evolution reaction (HER). This will be described later with
reference to experiment results of the inventors.
[0048] In addition, the surface of the tungsten carbide nanoflake
is partially covered by a protection film to complete the
manufacture of a structure for use as an electrode (S3).
[0049] FIG. 2 is a schematic view showing an electrode for hydrogen
production using a tungsten carbide nanoflake according to an
embodiment.
[0050] Referring to FIG. 2, an electrode body 20 of this embodiment
is disposed on a substrate 200. The substrate 200 may be a silicon
(Si) substrate, without being limited thereto. The electrode body
20 is electrically connected to a contact portion 21 made of
conductive material for the connection to an external power source
or a load. Further, the surface of the electrode body 20 may be
partially covered by a protection film 21. The electrode body 20 is
generated by the process described above with reference to FIG. 1,
and includes a NCD film and tungsten carbide of a two-dimensional
nanostructure grown from the NCD film. In addition, the surface of
the electrode body 20 not covered by the protection film 21 is
obtained by removing the oxide layer and/or the graphene layer from
the surface of the tungsten carbide by means of the cleaning
process described above with reference to S2 of FIG. 1.
[0051] Hereinafter, an experimental example of the method for
manufacturing an electrode for hydrogen production using a tungsten
carbide nanoflake according to the present disclosure, performed by
the inventors, and experiment results will be described.
<Formation of a NCD Film on a Silicon Substrate >
[0052] A NCD film was composed on a silicon substrate using a
hot-filament chemical vapor deposition (HFCVD). Before performing
the HFCVD process, the silicon substrate was put into a methanol
solution in which NCD particles with an average diameter of 3 nm
were dispersed, and ultrasonic treatment was performed thereto to
disperse the NCD particles on the substrate. In the HFCVD process,
a mixed gas of CH.sub.4 5% and H.sub.2 95% was used as a precursor,
and this process was performed for 30 minutes with a substrate
temperature of 750.degree. C. and a pressure of 7.5 Torr in the
chamber. Through the HFCVD process, a NCD film with a thickness of
440 nm and a particle size of 10 to 15 nm was formed.
<Composing of a Tungsten Carbide Nanoflake >
[0053] Tungsten carbide of a nanowall structure was composed on the
prepared NCD film using a DC-PACVD device.
[0054] An anode was disposed in a chamber of the DC-PACVD device, a
tungsten disc serving as a substrate holder is provided on the
anode, a substrate having the NCD film was provided on the tungsten
disc, and a surface-carburized tungsten cathode is disposed at a
location upwardly spaced apart from the substrate by 5 mm. The
surface-carburized tungsten cathode was formed in advance by
exposing the tungsten cathode to a diamond composing condition
(H.sub.2-CH.sub.4 precursor) using CVD, and a WC.sub.x structure is
formed to a predetermined depth from the surface of the
surface-carburized tungsten cathode.
[0055] The tungsten carbide was composed by applying a first
process condition and a second process condition. The first process
condition has a discharge voltage of 480V, a discharge current of
50 A, a cathode temperature of 600.degree. C., and a substrate
temperature of 600.degree. C., respectively. The second process
condition has a discharge voltage of 473V, a discharge current of
50 A, a cathode temperature of 800.degree. C., and a substrate
temperature of 800.degree. C., respectively. At both of the first
process condition and the second process condition, the chamber
pressure was 100 Torr, and the process time was 6 hours. Also,
hydrogen of 150 SCCM was supplied into the chamber to generate
hydrogen plasma. The hydrogen plasma played a role of moving
tungsten atom and carbon atom of the surface-carburized tungsten
cathode and the NCD to the growth front of the tungsten
carbide.
<Removal of an Oxide Layer and/or a Graphene Layer by Cleaning
>
[0056] The surface of the tungsten carbide nanoflake composed on
the NCD film as described above was treated by means of a cleaning
process with several thousand cycles to remove the oxide layer
and/or the graphene layer on the tungsten carbide nanoflake, and
HER activity was increased to enhance the usage as a
hydrogen-generating electrode. For the cleaning, 0.5 M sulfuric
acid solution was used. Here, a voltage was applied to the tungsten
carbide nanoflake immersed in the sulfuric acid solution, and the
applied voltage was changed from -0.3 V to 0 V and then from 0 V to
-0.3 V, which corresponds to one cycle. In the cleaning process,
4000 cycles were performed. At each cycle, a voltage change rate
was 10 mV/s. However, the cleaning process may also be performed in
a different way, and the cleaning solution, the voltage range and
the voltage moving rate are not limited to the above.
[0057] FIG. 3 is a graph comparatively showing linear sweep
voltammetry (LSV) curves of the tungsten carbide nanoflake composed
according to embodiments and a platinum (Pt) electrode, in which an
ohmic potential drop in comparison to a difference of an electrode
potential (E) and a real hydrogen electrode (RHE) potential is
exhibited. Graphs 301 and 302 and respectively represent LSV curves
of a nanowall structure before and after cleaning. Graphs 303 and
304 and respectively represent LSV curves of a nanocrystal
structure before and after cleaning. Graph 305 represents the LSV
curve of a platinum. Here, the increase of HER activity after
cyclically cleanings as much as 4000 cycles is shown. Even though
the activity increases by means of cleaning in both the nanowall
structure and the nanocrystalline structure, the activity
particularly increases greatly in the nanowall structure. The above
results are shown in Table 1 below.
TABLE-US-00001 TABLE 1 onset (Onset) overpotential current density
potential @ 10 mA/cm.sup.2 @ 0.25 V Material (V) (mV) (mA/cm.sup.2)
WC nanocrystalline -0.102 248 -10.31 WC nanowall -0.052 160 -40.10
platinum (Pt) -0.005 95 -83.85
[0058] The nanowall sample was excellent in onset potential and
current density. As described above, the difference in HER reaction
may be found through X-ray photoelectron spectroscopy. FIGS. 4A to
4D show an X-ray photoelectron spectroscopy (XPS) spectrum of a
tungsten carbide nanoflake composed according to embodiments. Here,
FIG. 4A shows tungsten carbide of a nanowall structure before
cleaning, FIG. 4B shows tungsten carbide of a nanowall structure
after cleaning, FIG. 4C shows tungsten carbide of a nanocrystalline
structure before cleaning, and FIG. 4D shows tungsten carbide of a
nanocrystalline structure after cleaning.
[0059] As shown in FIGS. 4A and 4B, it may be found that an oxide
peak is present at the surface of the initially composed tungsten
carbide nanoflake with a greater intensity in comparison to a
carbide peak, but after cleaning, the oxide peak decreases greatly
and the carbide peak remains as it was.
[0060] Meanwhile, in an embodiment of the present disclosure, by
means of cleaning, not only oxide but also the amount of a graphene
layer (or, a graphitic carbon layer) present at the surface of
tungsten carbide is reduced. The graphene layer present at the
surface of tungsten carbide is known as giving a bad influence on
electrochemical activity.
[0061] FIGS. 5A to 5D show an XPS spectrum of a tungsten carbide
nanoflake composed according to other embodiments. Here, FIG. 5A
shows tungsten carbide of a nanowall structure before cleaning,
FIG. 5B shows tungsten carbide of a nanowall structure after
cleaning, FIG. 5C shows tungsten carbide of a nanocrystalline
structure before cleaning, and FIG. 5D shows tungsten carbide of a
nanocrystalline structure after cleaning.
[0062] As shown in FIGS. 5A to 5D, it is found that the intensity
of a peak corresponding to the graphene layer is significantly
reduced after the tungsten carbide nanoflake is cleaned. In other
words, in embodiments of the present disclosure, the cleaning
process plays a role of removing the oxide and graphene layers from
the surface of the tungsten carbide nanoflake, and thus it is found
that the activity of the tungsten carbide nanoflake is increased as
the carbide is exposed as above.
[0063] FIG. 6 shows a scanning electron microscope (SEM) image of a
tungsten carbide nanowall film composed according to an embodiment.
Here, FIG. 6(A) is a planar image before cyclic cleaning, FIG. 6(B)
is a planar image after cyclic cleaning, and FIGS. 6(C) and 6(D)
are sectional images corresponding to FIGS. 6(A) and 6(B),
respectively.
[0064] The increase of HER activity at the tungsten carbide
nanoflake is caused by removal of the oxide and carbon layers, and
it may be regarded as being caused by dissolution of sulfuric acid
at the initial cyclic cleaning stage. FIG. 6 shows such a change,
and it is found in FIGS. 6(A) and 6(B) that the oxide layer and the
graphene layer are removed from the surface of the tungsten carbide
nanoflake by means of the cyclic cleaning process to reduce flakes,
respectively, and it is also be found in the sectional images of
FIGS. 6(C) and 6(D) that the surface thickness is reduced.
[0065] FIGS. 7A to 7D shows an XPS spectrum of a tungsten carbide
nanoflake composed according to other embodiments, and FIGS. 8A and
8B is a graph showing polarization curves according to the
embodiments of FIGS. 7A to 7D. In particular, FIG. 8A shows
polarization curves 801, 811, 821, 831 obtained after the initial
cyclic cleaning and polarization curves 802, 812, 822, 832 obtained
after performing the cyclic cleaning as much as 10,000 cycles for
the stability test, respectively. FIGS. 7A and 7B show results of
the nanowall structure, and FIGS. 7C and 7D show results of the
nanocrystalline structure. Also, FIG. 8A shows a result of the
nanowall structure, and FIG. 8B shows a result of the
nanocrystalline structure.
[0066] Referring to FIG. 7A to FIG. 8B, in the nanocrystalline
structure, degradation is severe, for example overpotential changes
by 14 mV at a current density of 10 mA/cm.sup.2 as shown in FIG.
8B, but in the nanowall structure, it may be found that activity is
not seriously changed at the stability test where the cleaning
cycles are extremely increased. The nanowall structure has high
stability as described above because the nanowall structure has a
small change in the intensity of oxide peak after the initial
cyclic cleaning (FIG. 7A and FIG. 4B), but the nanocrystalline
structure has a great difference in the oxide peak (FIG. 7C and
FIG. 4D). In both structures, the graphene peaks are similar (if
FIG. 5B and FIG. 7B are compared with FIG. 5D and FIG. 7D), and
thus, the high durability of the nanowall structure is regarded as
being caused by high resistance against oxidation in the HER
environment.
[0067] FIG. 9 is a high-resolution transmission electron microscope
(HR-TEM) image of a tungsten carbide nanowall composed according to
an embodiment, after cleaning.
[0068] It is known that the high oxidation resistance of tungsten
carbide is improved by means of crystalline perfection, as shown in
the HR-TEM image of FIG. 9. Referring to FIG. 9, it may be found
that, after cleaning, there is no unordered grain boundary at the
surface of the tungsten carbide having a nanowall structure, and
also the nanowall surface extends from the inside so that the {001}
crystal face is arranged substantially in parallel to the nanowall
surface to have high crystallinity. Meanwhile, in the
nanocrystalline structure, the grain boundary is relatively not
ordered, and thus it is known that, if the cleaning cycle is
elongated, the nanocrystalline structure is more likely to be
oxidized in comparison to the nanowall structure.
[0069] FIGS. 10A to 10B shows a Tafel plot and a turnover frequency
(TOF) of a tungsten carbide nanoflake composed according to
embodiments.
[0070] The result depicted in FIGS. 10A and 10B is obtained by more
quantitatively analyzing HER activity of the tungsten carbide
nanoflake. Here, FIG. 10A shows Tafel plots 1001, 1002 and 1003 of
a WC nanowall structure, a WC nanocrystal structure and a platinum,
respectively, and a Tafel slope calculated therefrom. FIG. 10B
shows a TOF calculated from the result of FIG. 10A. Graphs 1011 and
1012 respectively represent the TOF of a WC nanowall structure and
a WC nanocrystal structure, calculated from cyclic voltammetry (CV)
curve. Further, Graphs 1013 and 1014 respectively represent the TOF
of a WC nanowall structure and a WC nanocrystal structure,
calculated from observation results using a atomic force microscope
(AFM). The Tafel slope represents an amount of overvoltage required
for generating a unit current density, and the current density
means a reacting amount. If the Tafel slope is great, this means
that a voltage should be further applied to make the same reacting
amount, and thus this means that the HER activity is bad. In
addition, since the TOF means an amount of hydrogen molecules
produced per second, and thus as the TOF is greater, the HER
activity is more excellent.
[0071] As shown in the figures, it is found that the catalytic
activity of the tungsten carbide nanoflake composed according to
the embodiments, which is represented by the Tafel slope in FIG.
10A and the TOF in FIG. 10B, is greatly excellent in comparison to
other electrode materials known in the art. In particular, the
tungsten carbide of a nanowall structure is most excellent in onset
potential and has no overpotential degradation after a lot of
cycles.
[0072] The comparison results of catalytic activity between the
tungsten carbide nanoflake according to the embodiments and other
various materials are shown in Table 2 below.
TABLE-US-00002 TABLE 2 Tafel onset stability slope potential
potential overpotential material (mV/dec) (V) solution range cycle
changed Fe--WCN 47.1 -0.1 pH 1 -0.3 to 3000 none nanoparticle
H.sub.2SO.sub.4 0.5 V WC nanoparticle/ -- -- 0.5M -0.3 to 3000 10
mV carbon black H.sub.2SO.sub.4 0.6 V increased WC nanoparticle/
122 -- pH 1 -- 1000 none carbon nanotube H.sub.2SO.sub.4 (CNT) WCN
65 -- 0.1M -0.5 to 10000 19 mV nanoparticle HClO.sub.4 0.3 V
increased (nanocrystal) WC nanoparticle 84 -0.1 0.5M -0.3 to 800
none H.sub.2SO.sub.4 0.1 V W.sub.2C microsphere 118 -0.05 -- -- --
-- common WC 73 -0.1 -- -- -- -- powder WC nanocrystalline 83
-0.102 0.5M -0.5 to 10000 14 mV H.sub.2SO.sub.4 0.2 V increased WC
nanowall 67 -0.052 0.5M -0.5 to 10000 none H.sub.2SO.sub.4 0.2
V
[0073] Using the method for manufacturing an electrode for hydrogen
production using a tungsten carbide nanoflake according to an
embodiment, since an oxide layer and/or a graphene layer of a
surface of tungsten carbide is removed by means of cyclic cleaning
after tungsten carbide is formed, hydrogen evolution reaction (HER)
activity of the tungsten carbide may be increased, thereby
enhancing utilization as a catalyst electrode.
[0074] In addition, using the method for manufacturing an electrode
for hydrogen production according to an embodiment, tungsten
carbide of a two-dimensional nanostructure may be obtained by
combining a hydrogen plasma applying process and a chemical vapor
deposition process, and the degree of supersaturation on a growth
front may be adjusted to selectively change a geometric shape of
the tungsten carbide. Therefore, it is possible to manufacture
tungsten carbide of a nanowall or nanocrystalline structure, which
may be applied to various technical fields.
[0075] The present disclosure can be changed and modified in
various ways within the scope of the present disclosure by those
having ordinary skill in the art and thus is not limited to the
above embodiments and the accompanying drawings.
* * * * *